Objective To improve the corrosion resistance of AZ91D magnesium alloy and expand the range of application. Methods Ni-P coating was first prepared on AZ91D magnesium alloy by electroless plating, then Ni-Sn-P coating was prepared on this basis, forming Ni-P/ Ni-Sn-P duplex coatings. The microstructure and corrosion resistance of Ni-P/ Ni-Sn-P duplex coatings were investigated and compared with those of Ni-P coating. Results The results showed that the surface of Ni-P/ Ni-Sn-P duplex coatings was more uniform, compact, with fewer defects, lower porosity and amorphous structure. The corrosion potential of the secondary Ni-Sn-P coating was about -0. 77 V, which was lower than that of the first Ni-P coating ( -0. 68 V). The potential difference between the two coatings formed micro etching point, with Ni-P coating as the cathode, and Ni-Sn-P coating as the anode, and the anode was corroded first. Conclusion The outer layer of Ni-Sn-P coating in the Ni-P/ Ni-Sn-P duplex coatings could protect the first Ni-P coating from corroding and diffuse the corrosion current in the horizontal direction, thus greatly enhancing the corrosion resistance of AZ91D magnesium alloy.